American bacteriologist Theobald Smith showed that bacteria killed by heat could still confer immunity, and mixed diphtheria toxin with antitoxin to immunize animals against diphtheria, leading to a diphtheria immunization for children. He discovered Babesia bigemina, the tick-borne parasite that causes Texas cattle fever, which was the first proof that disease could be carried by insects, a finding which opened new avenues of research leading to breakthroughs against malaria, Rocky Mountain spotted fever, tularemia, typhus fever, yellow fever, and dozens of other diseases. Among his other significant work, he established that human and bovine tuberculosis are two different diseases, improved laboratory production of vaccines, and discovered anaphylaxis (a life-threatening allergic reaction) and serum sickness (a reaction to animal antiserums).

Author of books:Special Report on the Cause and Prevention of Swine Plague (1891)Investigations into the Nature, Causation, and Prevention of Texas or Southern Cattle Fever (1893)Investigations concerning Infectious Diseases among Poultry (1895)Studies in Vaccinal Immunity towards Disease of the Bovine Placenta due to Bacillus Abortus (Infectious Abortion) (1923, with Ralph B. Little)Parasitism and Disease (1934)